The present invention relates to an ultrasonic motor and more particularly to an ultrasonic motor in which a flexural vibrator and a longitudinal vibrator constituted by electro-mechanical transducing elements are combined together, the longitudinal vibrator being mounted to a node of flexural vibration of the flexural vibrator, and a driven member is moved by an ultrasonic driving force generated by both vibrators.
Ultrasonic motors have recently occupied the attention of people concerned. As is well known, this type of motor has the are characteristic in that a low-speed drive is easy and a high torque output is obtained. Some of them have already been commercialized. However, the commecialized ultrasonic motors are rotary type motors, and small-sized linear type motors having a high degree of design freedom and capable of being easily modified into a rotary type motor have not been commercialized yet. This is because it is difficult to manufacture a small-sized linear type ultrasonic motor of high efficiency and high output. For example, in a travelling wave type linear ultrasonic motor proposed in Japanese Patent Laid Open No. 96881/84, it is necessary to use a closed loop-shaped flexural vibrator for generating a travelling wave efficiently. Consequently, the vibrator becomes very large in size and the mechanism for supporting the vibrator is complicated in structure. Further, the motor efficiency itself is only several percent and thus not always high. In Japanese Patent Laid Open No. 277477/88 there is proposed a linear type ultrasonic motor in which a flexural resonance vibration of a plate and a longitudinal resonance vibration thereof are combined together. In this case, in order to realize a high efficiency motor it is necessary to make the resonance frequency in the flexural resonance of the plate and that in the longitudinal resonance thereof coincident with each other. Thus, restrictions are imposed on the size and shape, and hence it has been not easy to attain the reduction of size and high efficiency. In the motor in question, moreover, it is impossible to increase the pressure-contacting force and hence impossible to generate a large driving force because the vibration for generating a frictional force utilizes a transverse effect (the vibrating direction is orthogonal to a voltage application direction) of a piezoelectric element.
Further, an ultrasonic motor which utilizes nodes of a standing wave to drive a moving member as a contact member is disclosed, for example, in Japanese Patent Laid Open No. 69472/88. In this ultrasonic motor, projections projecting in the normal direction are disposed at node portions of a standing wave on one-side circumference portion of a disc-like vibrator which generates a standing wave of thickness flexural vibration, while on the other side of the vibrator there is disposed means for vibrating every other projection at opposite phases in the thickness direction. When a rotor is brought into pressure contact with each projection, the inclination in the circumferential direction of the projection is changed by the standing wave, and this change of the said inclination is utilized to impart a rotating force in a certain direction to the rotor. In the above prior art references there also is proposed a construction in which a plurality of longitudinal vibrators utilizing the transverse effect are mounted in the positions of nodes of a disc-like flexural vibrator.
On the other hand, as is well known, an ultrasonic motor is attracting the attention of many concerns as an actuator which permits a focusing operation of a photographic lens of a camera, for example, in a small space. This is for the following reason. Ultrasonic motors, as compared with conventional electromagnetic motors, can afford a large torque at low speed, so a lens holding frame can be driven directly, that is, a reduction gear is not needed, and it is possible to realize the driving of a lens at high accuracy, high response, reduced size and without noise.
In the case of using an ultrasonic motor as an actuator for driving a lens of a camera, some consideration is needed about the shape and position of the actuator for disposing the actuator in a small space within a hollow cylindrical frame containing a lens holding frame. This has been realized by an annular ultrasonic motor using a travelling wave which is disclosed in Japanese Patent Laid Open No. 222672/89. According to this ultrasonic motor, a travelling wave is generated in the circumferential direction of a ring-shaped vibrator and an end face of a cylindrical body serving as a rotor is brought into pressure contact with an end face at which the travelling wave is generated to rotate the cylindrical body; further, for converting the rotation of the cylindrical body to a rectilinear motion of a lens, the cylindrical body is provided with a conversion mechanism such as a cam or a helicoid screw, whereby the lens is moved forward and backward in the optical axis direction.
There has also been proposed such a construction as disclosed in Japanese Patent Publication No. 7245/85 in which a lens is provided with inch worm type actuators whereby the lens is driven rectilinearly. More particularly, two actuators for clamping and one actuator for driving are operated successively to drive a lens holding frame rectilinearly.
Further, according to the construction disclosed in Japanese Patent Laid Open No. 99714/87, a travelling wave type rail-shaped rectilinear motor is disposed on the outer periphery of a lens holding frame and the lens holding frame is driven by a travelling vibration wave generated in the motor.
In the construction proposed in the foregoing Japanese Patent Laid Open No. 69472/88, it is necessary that projections disposed on nodes of a first vibrator be vibrated in the thickness direction by a flexural vibration generated at a second vibrator in synchronism with a flexural vibration generated by the first vibrator. To this end it is necessary to make the resonance frequency of the first vibrator and that of the second vibrator coincident with each other, and hence a high level of technique is required for the manufacture of the vibrators. Besides, even when the first vibrator is vibrated, the vibration of the first vibrator is suppressed by the second vibrator disposed adjacent the first vibrator. Conversely, the vibration of the second vibrator is suppressed by the first vibrator. Thus, loss of vibration energy is large.
In Japanese Patent Laid Open No. 69472/88 there is disclosed means using a longitudinal vibrator which utilizes the transverse effect of a piezoelectric element on a node of a flexural disc. In this case, for driving at an amplitude required for driving a moving member, there may be adopted any of the following means:
1 Applying a high voltage signal to the longitudinal vibrator.
2 Making the longitudinal vibrator longer.
3 Resonating the longitudinal vibrator.
However, many problems are involved in practising these means, which problems will be explained below with reference to FIG. 19.
FIG. 19 is a perspective view for explaining the operation of a longitudinal vibrator indicated at 101. In the same figure, when voltage is applied to the longitudinal vibrator 101 in the thickness direction for utilizing the transverse effect of a piezoelectric element, if the original length, thickness and width of the longitudinal vibrator are L (m), T (m) and W (m), respectively, a piezoelectric distortion constant in the length L direction is d.sub.31 (m/V) and an applied voltage is V (v), then a displacement .DELTA.L (m) is expressed by the following relationship: EQU .DELTA.L=d.sub.31 (L/T)V (1)
In the case where 100 V is applied to a longitudinal vibrator having a length of 10 mm and a thickness of 2 mm, V is -100 V because the voltage application direction is opposite to a polarization direction A.sub.1. If the piezoelectric distortion constant d.sub.31 is set to d.sub.31 =-130.times.10.sup.-12 m/V which is a standard value of a piezoelectric element material used in ultrasonic motors, the displacement .DELTA.L (m) is given as follows: ##EQU1## Thus, even under the application of negative 100 V, the displacement .DELTA.L (m) is as small as 0.065 .mu.m. In this connection, studies will be made below about means for increasing the displacement .DELTA.L.
First, a study will be made about enlarging the displacement .DELTA.L by increasing voltage as in the foregoing item 1. For example, where a displacement of 1 .mu.m is to be obtained, .DELTA.L=0.065 .mu.m at V=100 V in the above equation (1a), so 1/0.065=15, that is, a very high application voltage of 1,500 V corresponding to fifteen times the voltage of 100 V is required.
Next, a study will be made about obtaining a sufficient displacement .DELTA.L=1 .mu.m by making the longitudinal vibrator longer as in the foregoing item 2. If the above equation (1) is changed in form to determine the original length L of the longitudinal vibrator, the length L is given as follows: ##EQU2## Substitution of actual numerical values into the above equation gives: ##EQU3## That is, a longitudinal vibrator as long as 15.3 cm is required for obtaining a displacement of 1 .mu.m.
Further, a study will now be made about the utilization of resonance as in the foregoing item 3. If a frequency constant in the longitudinal direction is N.sub.2 (Hz-m) and the length is L (m), a resonance frequency, f.sub.r, (Hz), in the longitudinal direction is expressed by the following relationship: EQU f.sub.r =N.sub.2 /L (2)
This equation (2) can be transformed as follows: EQU L=N.sub.2 /f.sub.r ( 2a)
If the value 1570 (Hz-m) of the material commonly used in ultrasonic motors, as the longitudinal frequency constant N.sub.2, and a general driving frequency 40 KHz of ultrasonic motors, as the longitudinal resonance frequency f.sub.r of the longitudinal vibrator, are substituted into the above equation (2a), the length L of the longitudinal vibrator is given as follows: EQU L=1570/40000=0.03925 (m)
That is, the length L of the longitudinal vibrator which resonates at 40 KHz is about 4 cm, corresponding to about one fourth of 15.3 cm which has been obtained in the study of the foregoing item 2. But even this length is still large as the length of the longitudinal vibrator. In such a longitudinal vibrator about 4 cm in length, a flexural resonance point exists at a point lower in frequency, so the operation of longitudinal vibration becomes unstable and impractical, and the piezoelectric element itself becomes easier to be broken.
In the case where a large number of longitudinal vibrators are arranged on a disc, it is necessary that the front ends of the longitudinal vibrators be contacted uniformly with the rotor in order to transfer driving forces generated by the longitudinal vibrators effectively to the rotor. Consequently, it becomes difficult to maintain the flatness of the contact surface of each longitudinal vibrator.
For driving a lens by the foregoing annular ultrasonic motor using a travelling wave disclosed in Japanese Patent Laid Open No. 222672/89, it is necessary to provide a conversion mechanism, e.g. helicoid screw or cam, for converting a rotational motion generated in the motor to a rectilinear motion. And a hollow motor is large in size and complicated in structure. Moreover, the use of a helicoid screw or a cam causes mechanical wobbling, thus making it difficult to attain a high positional accuracy of the lens holding frame.
In the foregoing Japanese Patent Publication No. 7245/85 using an inch worm type actuator for driving a lens, the operational mechanism disclosed therein permits driving only up to a step frequency of several KHz or so, and thus there is the drawback that the operating speed is low. Further, it is necessary for the clamping actuators to fix and hold the lens frame to a stationary frame during clamping and be completely spaced away from the stationary frame when clamping is not made. As the clamping actuators there are used piezoelectric laminates having an operating stroke of several to several tens of micron meters. Wear of the clamped surface may cause the clamping operation to become unstable, sometimes resulting in discontinuance of the driving of the lens. Moreover, for holding the lens in a certain position it is necessary to continue the application of power to the actuators and thereby keep the actuators clamped. This means energy consumption even when operation is not performed.
In the travelling wave type linear ultrasonic motor disclosed in the foregoing Japanese Patent Laid Open No. 99714/87, a closed loop-shaped vibrator for generating a travelling wave must be disposed at the outer periphery of lens, so if a lens frame is used, it is required to be larger in size. Besides, since a travelling wave is used, it is difficult to fix the vibrator without impairment of vibration, and if the vibrator is supported through a damping material or the like, the lens position in the direction orthogonal to the optical axis of the lens with respect to the stationary member is not determined accurately.